The present invention relates generally to the field of air separation and has particular reference to the crude recovery of at least one rare gas selected from the group consisting of krypton and xenon from an oxygen product of an air separation.
Krypton and xenon are present in air at very low concentrations, typically about 1.14 parts per million (xe2x80x9cppmxe2x80x9d) and about 0.087 ppm respectively. They are both valuable gases and, thus, there is an economic incentive to maximise their recovery in an air separation process.
In typical cryogenic air distillation processes, krypton and xenon concentrate in the liquid oxygen (xe2x80x9cLOXxe2x80x9d) product taken from the bottom of the low pressure (xe2x80x9cLPxe2x80x9d) distillation column as they are far less volatile than oxygen. The smaller the LOX flow, therefore, the more concentrated the krypton and xenon in this product.
In cryogenic air distillation processes in which most of the oxygen product is removed from the LP column in the gas phase, it is possible to make sure that very little krypton and xenon is lost in the gaseous oxygen (xe2x80x9cGOXxe2x80x9d) by removing the GOX several distillation stages above the bottom of the LP column. These bottom guard stages are used mainly to prevent excessive loss of krypton which is substantially more volatile than xenon. Almost all of the krypton and xenon entering the air separation plant can then be recovered in the LOX product, which is a very small proportion of the total oxygen flow. This LOX product can then be processed to produce a purified rare gas product. In the event that it is primarily a xenon product that is required, one could dispense with the bottom guard stages and still recover much of the krypton and almost all of the xenon entering the plant in the LOX product.
If the LOX flow from the distillation process is much greater, for example when all the oxygen is withdrawn from the distillation column as LOX, pumped to the required pressure and evaporated in the main heat exchanger, the loss of krypton and xenon is much greater, even when the LOX is taken several stages up the LP column, separately from the liquid stream in which is concentrated the rare gas components. Essentially all of the krypton and xenon entering the air separation plant flows down the LP column to the sump of the LP column in the descending liquid, so any liquid withdrawal will remove a portion of the krypton and xenon proportional to the total liquid withdrawn as product. This will typically lead to losses of about 30% of these valuable products.
It is desirable, therefore, to increase the recovery of krypton and xenon from an air separation plant in which at least part of the oxygen product is withdrawn as LOX.
When a plant withdraws the main oxygen product as a vapor from the LP column, the krypton and xenon can be recovered by processing the LOX product as described above. However, for existing pumped LOX cycle plants, there is usually no small stream having concentrated rare gas components as all oxygen products are generally withdrawn from the bottom of the LP column. Therefore, as krypton and xenon are so valuable, it is also desirable to be able to retrofit rare gas recovery systems to existing plants.
In addition, it is desirable to provide a xenon and/or krypton recovery plant, which can process rare gas-enriched feed streams from an external source.
U.S. Pat. No. 4,805,412 (Colley; published on Feb. 21, 1989) discloses a process and apparatus for the cryogenic distillation of air with reduced loss of krypton and xenon. Oxygen is withdrawn from the LP column of the distillation system and is fed to a primary krypton column for extraction of its krypton and xenon content. The main feed to the primary krypton column is a stream of LOX but a small stream of GOX is also taken from the LP column and is fed without pressure adjustment to the krypton column. The LP column and the primary krypton column operate at substantially the same pressure. A portion of the krypton-lean overhead vapor is condensed and fed to the primary krypton column as descending wash liquid.
U.S. Pat. No. 6,301,929 (Lochner; published on Oct. 16, 2001) discloses an air separation process in which a rare gas-lean LOX stream and a rare gas-enriched LOX stream are formed. The two liquid streams are pumped to a rare gas separation column operating at GOX product pressure. The rare gas-lean LOX stream is passed as reflux to the top of the column and the rare gas-enriched stream is passed to a lower section of the column. Rare gas-lean GOX product is withdrawn as overhead from the column and a further rare gas-enriched bottoms liquid stream is withdrawn. The reboiler/condenser in the sump of the column is sized to vaporize almost all the oxygen feed streams. As the oxygen feed streams are liquid, the reboiler/condenser must be large to vaporize all of the feed.
Research Disclosure No.42517 (disclosed anonymously in September 1999) discloses an air separation process in which the oxygen product is removed from the column system as LOX. The LOX stream is pumped to the oxygen product pressure and divided into two steams. The first stream is passed as reflux to the top of a rare gas column and the second stream is passed to a lower zone of the column. The relative proportions of the two streams are determined such that the column can reject methane. Rare gas-lean GOX product is withdrawn as overhead from the rare gas column and a rare gas-enriched bottoms liquid stream is withdrawn. The reboiler/condenser in the sump of the rare gas column must be sized to vaporize almost all the oxygen feed streams. As the oxygen feed streams are liquid, the reboiler/condenser must be large to vaporize all of the feed.
DE-A-19855485 (Lochner; published on Jun. 10, 1999) discloses an air separation process in which rare gas-lean LOX and rare gas-enriched LOX are formed in the LP column. The two liquid streams are pumped to a rare gas column, the lean stream being passed as reflux to the top of the column and the enriched stream being passed to a lower section of the column. In addition, some gaseous nitrogen (xe2x80x9cGANxe2x80x9d) is added to the bottom of the rare gas column to strip liquid descending the column. Rare gas-lean GOX overhead is returned to the LP column and a further rare gas-enriched LOX stream is withdrawn.
U.S. Pat. No. 6,378,333(Dray; published on Apr. 30, 2002) discloses an air separation process in which a first LOX stream having a xenon component is passed from the LP column to the upper portion of a xenon concentrator column as reflux. In the xenon concentrator column, the LOX feed is separated into xenon-rich bottoms liquid and xenon-lean GOX overhead. A second LOX stream having a xenon component is withdrawn from LP column, pressurized and partially vaporized against a portion of feed air. Typically, the liquid fraction from this partial vaporization will be also passed as feed to the xenon concentrator column.
U.S. Pat. No. 5,913,893 (Gary et al; published on Jun. 22, 1999) discloses a method of purification of a cryogenic fluid, especially liquid helium, by filtration and/or adsorption. The impurities are filtered/adsorbed from the fluid but are not available as a valuable product.
U.S. Pat. No. 5,039,500 (Shino et al; published on Aug. 13, 1991) discloses the gasification of a small LOX purge stream taken from an air separation unit (xe2x80x9cASUxe2x80x9d) and passing the gaseous stream through an adsorber which selectively adsorbs xenon. Xenon is recovered during the regeneration phase of the adsorber. The xenon concentration in the purge stream is about 31 ppm, i.e. about 360 times the concentration of xenon in air.
JP-A-09002808 (Takano et al; published on Jan. 7, 1997) discloses the gasification of a small LOX purge stream taken from an ASU and passing the gaseous stream through a first adsorber (which selectively adsorbs xenon) and then through a second adsorber (which selectively adsorbs krypton). Xenon and krypton are recovered during the regeneration phase of the adsorbers.
It is well known in the art that krypton and xenon will concentrate in oxygen liquid because of the extremely low volatility of these gases. Thus, it is a requirement in the prior art that a LOX stream be processed in order to recovery krypton and xenon. Most of the prior art additionally provides a small oxygen purge stream concentrated in krypton and xenon so that the crude recovery system will be smaller. There is no disclosure in the aforementioned prior art of the recovery of krypton and xenon from a warm product gaseous oxygen stream.
In prior art processes, if krypton and xenon recovery is required, it is generally necessary to design the LP column of an ASU so that a small rare gas-rich LOX purge can be withdrawn. Such modifications add significantly to the necessary capital investment and to the height to the LP column.
It is desirable to overcome disadvantages of (and thereby improve on) the exemplified prior art and to provide an air separation process that is able to produce a rare gas-enriched product (for further processing into purified krypton and/or xenon products) and a pure LOX product without involving the capital expense and running costs of a large reboiler/condenser or additional equipment such as an argon stripping column.
The inventors have realized that crude xenon recovery can be achieved by contacting xenon- (and usually krypton-) containing vapor feed with a reflux liquid, even if the vapor feed has a low concentration of krypton and xenon and if the vapor feed is at a high pressure. Krypton can also be recovered. The recovered product may then be further processed to provide at least one purified krypton and/or xenon product. The expression xe2x80x9clow concentrationxe2x80x9d used in the context of krypton and xenon in the vapor feed is intended to mean that the krypton and xenon concentration in the vapor feed is lower than in prior art purge streams but higher than in air.
According to a first aspect of the present invention, there is provided a process for the recovery of at least one rare gas selected from the group consisting of krypton and xenon from a mixture comprising oxygen and at least one rare gas selected from the group consisting of krypton and xenon. The process comprises feeding said mixture or a mixture derived therefrom to a rare gas recovery system and separating said mixture feed in said rare gas recovery system into rare gas-lean GOX and rare gas-enriched product. The process is characterised in that at least about 50 mol % of said mixture is fed to the rare gas-recovery system in the gaseous phase. When the mixture feed is separated by selective adsorption, the concentration of xenon in the mixture feed is no greater than 50 times the concentration of xenon in air.
The invention involves passing feed mixture, the bulk of which is oxygen and at least about half of which is gaseous, to a crude rare gas recovery system. The crude rare gas recovery system could be a column, a column system, a heat exchanger or an adsorber but, whatever the nature of the recovery system, krypton and/or xenon components are concentrated and the concentrated product (and purified oxygen product) recovered. Rather than processing a small purge stream rich in krypton and xenon, preferred embodiments of the invention are intended to process a larger GOX stream having a lower krypton and xenon concentration.
The feed is preferably at a pressure higher than that of the source from which it is taken, e.g. the cryogenic air distillation column from which it was originally withdrawn. The feed may be vaporized oxygen resulting from a pumped LOX cycle in an ASU, GOX from the warm-end of the main exchanger (possibly following compression) or may be from an oxygen pipeline. LOX may also be fed to the crude recovery system to provide refrigeration and/or reflux.
One advantage of the present invention is that it can be applied to existing plants producing oxygen. Such plants may be easily retrofitted with a crude recovery system according to the present invention such that the krypton and xenon in the existing oxygen product streams may be recovered without additional processing.
Preferably, at least 90 mol % of the mixture feed is gaseous. More preferably, all of the mixture feed is gaseous.
The process is applicable to the production of xenon-enriched product, krypton-enriched product and xenon- and krypton-enriched product.
The process may further comprise feeding a xenon-enriched stream to the rare gas-recovery system. The enriched stream may be at least partially gaseous or may be liquid and may be taken from a cryogenic air distillation system, which would usually be a different system to that generating the mixture.
The mixture may be taken from a GOX pipeline in which case it would usually be under pressure and may not further require pressurization. Alternatively, the mixture may be pressurized before being fed to the rare gas recovery system, for example, if it is removed from a LP column of an ASU.
The process may further comprise separating feed air in a cryogenic air separation unit (xe2x80x9cASUxe2x80x9d) into nitrogen-rich overhead vapor and liquid oxygen (xe2x80x9cLOXxe2x80x9d), pressurising at least a portion of said LOX to provide pressurized LOX and at least partially vaporizing at least a portion of said pressurized LOX to provide said mixture feed. In such processes, all of said at least a portion of said pressurized LOX is preferably vaporized to produce said mixture. The pressure of the mixture feed is preferably greater than the operating pressure of the part of the ASU producing said LOX. The LOX stream may be divided into at least two portions, each portion being vaporized at different pressures before being fed to the rare gas recovery system.
In one embodiment of the invention, the rare gas recovery system is a gas-liquid contact separation system and the process comprises contacting said mixture feed with LOX in the separation system to effect the separation.
In one arrangement of this embodiment, the gas-liquid contact separation system is a gas-liquid contact column with no distillation stages and the process comprises passing (e.g. bubbling) the mixture feed through LOX in said column to effect the separation.
In another arrangement, the gas-liquid contact separation system is a distillation system and the process comprises feeding said mixture to the distillation system for separation into said rare gas-lean GOX as overhead vapor and said rare gas-enriched product and feeding LOX to said distillation system as reflux. Preferably, the mixture is superheated prior to being fed to the distillation system.
Whether at least the majority of the mixture is fed to the distillation system in either gaseous or liquid state, the rare gas-lean overhead may be condensed, pressurised to a high pressure (e.g. using a pump) and then revaporised.
Where the gas-liquid contact separation system is a distillation system, the process may further comprise separating feed air in a cryogenic ASU into nitrogen-rich overhead vapor and LOX, removing a stream of LOX from the ASU, pressurizing at least a portion of said LOX stream to produce a stream of pressurized LOX, dividing said pressurized LOX steam into a major portion and a minor portion, at least partially vaporising said major portion to provide said mixture and feeding said minor LOX portion to the distillation system as reflux. All of said major portion is preferably vaporized to produce said mixture. Where the distillation system comprises a single distillation column, the process comprises feeding said mixture to the column for separation into said rare gas-enriched product and said rare gas-lean GOX and feeding said minor LOX portion to said column as reflux.
Where the gas-liquid contact separation system is a distillation system, the process may comprise separating feed air in a cryogenic ASU into nitrogen-rich overhead vapor and LOX, removing a stream of LOX from the ASU, pressurizing at least a portion of said LOX stream to produce a stream of pressurized LOX, at least partially vaporising at least a portion of said pressurized LOX steam to provide said mixture, condensing at least a portion of said rare gas-lean GOX overhead vapor by indirect heat exchange against a refrigerant to produce condensed overhead and feeding at least a portion of the condensed overhead to the distillation system as reflux. Preferably, all of said at least a portion of said pressurized LOX stream is vaporized to produce said mixture. If the distillation system comprises a single distillation column, then the process may comprise feeding said mixture to the column for separation into said rare gas-enriched product and said rare gas-lean GOX and feeding at least a portion of said condensed overhead to said column as reflux.
The distillation system may comprise one or more distillation columns. Usually, the system has only one column as this reduces the initial capital investment but systems having either two or three columns may be preferred in certain circumstances.
In one arrangement of this embodiment, the distillation system comprises at least a higher pressure (xe2x80x9cHPxe2x80x9d) distillation column and a lower pressure (xe2x80x9cLPxe2x80x9d) distillation column. The HP and LP columns are thermally integrated via a reboiler/condenser. The process comprises feeding said mixture to said HP column where it is separated into rare gas-depleted overhead vapor and rare gas-enriched bottoms liquid. Rare gas-enriched bottoms liquid is fed to said LP column after pressure adjustment for separation into said rare gas-lean GOX and said rare gas-enriched product. Rare gas-depleted overhead vapor is at least partially condensed by indirect heat exchange against rare gas-enriched product to produce at least partially condensed rare gas-depleted overhead, at least a portion of which is fed to the HP column as reflux. Liquid from or derived from the HP column is fed to the LP column as reflux. LOX may be fed to the HP column as reflux. LOX for reflux is typically produced by the separation of feed air in a cryogenic ASU. The HP column may be reboiled by at least partially vaporizing rare gas-enriched bottoms liquid by indirect heat exchange against a heating fluid.
In another arrangement of this embodiment, the distillation system comprises at least a higher pressure (xe2x80x9cHPxe2x80x9d) distillation column, a medium pressure (xe2x80x9cMPxe2x80x9d) distillation column and a lower pressure (xe2x80x9cLPxe2x80x9d) distillation column. The HP and MP columns are thermally integrated via a first reboiler/condenser and the MP and LP columns are thermally integrated via a second reboiler/condenser. The process comprises feeding said mixture to said HP column where it is separated into first rare gas-depleted overhead vapor and first rare gas-enriched bottoms liquid. First rare gas-enriched bottoms liquid is fed to said MP column after pressure adjustment for separation into second rare gas-depleted overhead vapor and second rare gas-enriched bottoms liquid. First rare gas-depleted overhead vapor is at least partially condensed by indirect heat exchange against second rare gas-enriched bottoms liquid to produce at least partially condensed first rare gas-depleted overhead, at least a portion of which is fed to the HP column as reflux. Liquid from or derived from the HP column is fed to the MP column, the LP column or both as reflux. Second rare gas-enriched bottoms liquid is fed to said LP column for separation into said rare gas-lean GOX and said rare gas-enriched product. Second rare gas-depleted overhead vapor is at least partially condensed by indirect heat exchange against said rare gas-enriched product to produce at least partially condensed second rare gas-depleted overhead, at least a portion of which is fed to the MP column as reflux. Liquid from or derived from the MP column is fed to the LP column as reflux. The process may further comprise feeding LOX to the HP column as reflux. The LOX for this reflux is preferably produced by cryogenic separation of feed air.
In a further arrangement of this embodiment, the distillation system comprises at least a first distillation column and a second distillation column with first and second columns operating at the same pressure. The process comprises feeding said mixture to said first column for separation into rare gas-lean GOX and rare gas-enriched product, feeding said mixture to said second column for separation into rare gas-lean GOX and rare gas-enriched product, feeding LOX to said first column as reflux and feeding at least one liquid selected from the group consisting of rare gas-enriched product from the first column and LOX to said second column as reflux. The process may further comprise dividing a stream of said pressurized mixture into two equal portions and feeding one portion to each column.
In another embodiment, the gas-liquid contact separation system is at least one heat exchanger. In such an embodiment, the process comprises feeding said mixture to the bottom of the or each heat exchanger, condensing a portion of said mixture ascending through the passages of the or each heat exchanger by indirect heat exchange against refrigerant to produce condensed mixture and contacting ascending mixture with descending condensed mixture in the passages to effect the separation by dephlegmation. Preferably, the indirect heat exchange takes place in the upper portion of the or each heat exchanger. The or each heat exchanger may be reboiled by at least partially vaporizing rare gas-enriched product by indirect heat exchange against a first heating fluid. The process may further comprise warming the rare gas-lean GOX to ambient temperature by indirect heat exchange within the or each heat exchanger against a second heating fluid, said heat exchange taking place above the heat exchange to produce the condensed mixture.
In a third embodiment, the rare gas recovery system is an adsorber system and the process comprises contacting said mixture feed with rare gas selective adsorbent material in the adsorber system to effect the separation. The process may be either a pressure swing adsorption (xe2x80x9cPSAxe2x80x9d) process or a temperature swing adsorption (xe2x80x9cTSAxe2x80x9d) process, both of which are well-known in the art.
The concentration of xenon in the mixture feed is no more than 50 times, preferably no more than 20 times and most preferably about 5 times, the concentration of xenon in air.
The separation is usually a crude separation and the rare gas-enriched product may be further processed to produce at least one product selected from the group consisting of a purified xenon product, a purified krypton product and a purified krypton and xenon product. Such further processing steps are well known in the art and include combusting the rare gas-enriched product to remove hydrocarbon compounds followed by further purification of the resultant product by distillation.
At least one further adsorber may be used to remove hydrocarbon compounds, carbon dioxide and/or nitrous oxide from LOX or GOX feed streams to the rare gas recovery system or from intermediate or final LOX or GOX streams.
According to a second aspect of the present invention, there is provided apparatus for the recovery of at least one rare gas selected from the group consisting of krypton and xenon from a mixture comprising oxygen and at least one rare gas selected from the group consisting of krypton and xenon according to the first aspect, said apparatus comprising a cryogenic ASU for separating feed air into nitrogen-rich overhead vapor and LOX, ressurizing means for pressurizing at least a portion of said LOX to provide pressurized LOX, vaporizing means for vaporizing at least about 50 mol % of said pressurized LOX to provide said mixture, and a rare gas recovery system for separating said mixture into rare gas-lean GOX and rare gas-enriched product.
The rare gas recovery system may be a gas-liquid contact column with no distillation stages. The mixture is passed (e.g. bubbled) through LOX in such a column.
The rare gas recovery system may also be a distillation system. Such apparatus may further comprise means for superheating said mixture prior to it being fed to the distillation system. The apparatus may further comprise conduit means for feeding a portion of said pressurized LOX from the pressurizing means (e.g. a pump) to the vaporizing means and further conduit means for feeding the remaining portion of said pressurized LOX from the pump to the distillation system as reflux. The apparatus may further comprise heat exchange means for at least partially condensing a portion of said rare gas-lean GOX overhead against a refrigerant to provide at least partially condensed rare gas-lean GOX overhead and conduit means for feeding at least a portion of said at least partially condensed overhead to the distillation system as reflux.
In a preferred embodiment where the rare gas recovery system is a distillation system, the apparatus may further comprise a first distillation column for separating mixture into rare gas-lean GOX and rare gas-enriched product, a second distillation column for separating mixture into rare gas-lean GOX and rare gas-enriched product at the same pressure as said first distillation column, conduit means for feeding LOX to the first distillation column as reflux and conduit means for feeding at least one liquid selected from the group consisting of rare gas-enriched product from the first distillation column and LOX to the second distillation column as reflux.
The rare gas recovery system may be at least one heat exchanger for separating the mixture by dephlegmation. The apparatus may further comprise first heat exchange means provided in the upper portion of the or each heat exchanger for condensing ascending mixture by indirect heat exchange against a refrigerant. The apparatus may further comprise second heat exchange means, e.g. provided in the lower portion of the or each heat exchanger, for vaporizing rare gas-enriched product by indirect heat exchange against a first heating fluid. The apparatus may further comprise third heat exchange means provided above the first heat exchange means in the or each heat exchanger for warming rare gas-lean GOX to ambient temperature by indirect heat exchange against a second heating fluid.